1. Field of the Invention
The present invention relates to inkless fingerprinting systems used for identification purposes and to unique solvents, color-formers, and developer combinations for such systems.
2. Background of the Art
The fingerprint patterns or ridge endings and ridge separations are highly individualized and are not altered with time. The unique character of the fingerprint of an individual has provided the basis for police identification of criminals, establishing the identity of accident victims, newly born babies, and numerous other situations and the comparison of fingerprint patterns has long been accepted as an absolute means of identifying individuals in a multitude of criminal and non-criminal situations.
Traditionally fingerprints have been made with printing or writing types of ink, usually comprising finely ground carbon black particles dispersed in a liquid vehicle. The most common method used to make fingerprints is to impregnate a pad with ink, transfer the ink to the surface of the object to be fingerprinted or identified, and subsequently transfer the ink to the surface of the substrate where the final print is to be made. Such a procedure is cumbersome, time consuming, and results in severe soiling of the hands and clothing of everyone involved in the fingerprinting process. This method also suffers from other drawbacks. If the fingers are wet with perspiration the image tends to blur and to lose resolution. This technique also requires great care and expertise to obtain good print.
In order for a fingerprint identification system to be commercially acceptable it must be extremely stable and reliable. The prints must be distinct and clear and must be easily readable by the human eye and by automated fingerprint reading systems which are finding increased use especially within law enforcement agencies. Furthermore the prints must form very rapidly and must possess a high degree of stability toward temperature, humidity, and light. The systems must be simple and aesthetically inoffensive. Inking methods suffer from the need to clean the finger of residual ink to prevent staining of objects handled after the fingerprint has been taken. This creates a disposal problem of the wipes used to clean the ink from the fingers. The cleaning operation is particularly objectionable when fingerprinting or footprinting babies.
Although improved techniques for fingerprinting which do not stain the finger have been developed they have not been widely adopted.
Draeger et al., U.S. Pat. No. 5,143,551 discloses a single use inking card for fingerprinting, with overlapping interconnected front and back sheets with an inked sheet in between.
Koch, U.S. Pat. No. 5,263,742 discloses an inkless fingerprinting system comprising a transparent sheet coated with a pressure sensitive adhesive film and an element for back-reflecting radiant energy for making a copy of the fingerprint.
Lougheed et al., U.S. Pat. No. 5,233,404 also discloses an inkless method of recording a fingerprint. A finger is placed on an illuminated surface and the image is projected onto a viewing surface. The image is then recorded with a charge coupled device which is rolled synchronously with the finger on the illuminated surface.
In yet another system for recording fingerprints, U.S. Pat. No. 4,699,077 teaches a pad impregnated with a color-former composition sealed in a packet and associated with a substrate carrying developer composition. The pad contains a polyhydroxy aromatic developer and the receptor contains the color-former. Exposure of sensitive skin to the these polyhydroxy aromatics is subject to question because of the nature of these compounds.
A fingerprint system based upon the formation of a colored image by the reaction of a transition metal salt with a dithiooxamide ligand is commercially available from 3M Company, St. Paul, Minn. under the name of "The Identifier." However, the color of the image produced is not black.
Additional inkless fingerprinting systems have been proposed. U.S. Pat. No. 3,831,552 discloses the use of magnetizable powders. U.S. Pat. No. 2,082,735 discloses the use of chelation of metal salts with organic acids. U.S. Pat. Nos. 3,960,632, 4,262,623, and 4,379,178 disclose the reaction of 8-hydroxyquinoline with metal salts such as ferric chloride. U.S. Pat. No. 4,232,083 discloses the use of metal complexing compounds having a plurality of ligand groups with transition metal salts of oleophilic organic acids to form dark images which can be useful in fingerprinting systems.
Vassiliades, U.S. Pat. No. 5,009,919 discloses a system for inkless fingerprinting using a color-former in an oleophilic solvent and a substrate coated on one surface with color developer. Vasiliades lists requirements for the solvent, specifically mentioning solvating properties for the color-former, low evaporation rate, good flow properties, and no adverse-toxicological effects. He then lists solvents such as alkylated phenyls, such as monoisobutyl biphenyl and monoisopropyl biphenyl; chlorinated paraffins; alkylated naphthalenes; partially hydrogenated terphenyls, such as Monsanto HB-40; natural vegetable oils such as soya-bean oil, cottonseed oil, and coconut oil; ester alcohols such as Eastman Kodak's Texanol.TM.; alkylated glycol ethers and ether acetates such as Eastman Kodak's Ektasolve.TM. series; and combinations thereof. Numerous developers for the color-former are described including common carbonless paper developers such as acid clays, phenolics, and salicylic acid; as well as color-formers obtained by augmenting the developer with additional metal developers. A variation on this method is also taught by Vassiliades in U.S. Pat. No. 4,879,134 wherein a fingerprint pad contains reacted color-former and developer. Here again, solvents, color-formers, and developers are those described in U.S. Pat. No. 5,009,919 above.
While these patents recognize the need for fast development of a fingerprint system that does not stain fingers, they use only the usual carbonless paper solvents. These solvents all suffer from specific deficiencies, such as objectionable odor, limited solubility of the leuco dye color-former (as in the case of the vegetable oils), toxicity (as in the case of the chlorinated paraffins), or irritation of the skin (as in the instances involving aromatic and hydrogenated aromatics).
One aspect of the importance of the solvent to image-forming properties, such as high solubility of the color-former, is the need for maintaining compatibility with sensitive skin. This issue was addressed in Hilterhaus et al., U.S. Pat. 4,859,650. Hilterhaus et al. were concerned with the preparation of a carbonless paper using leuco dye color-formers in capsules. The inventors used triphenylmethane leuco dyes with improved solubility. This allowed them to use a solvent composed of at least 80% of plant, animal, or paraffin oils. Their improved leuco dyes were soluble to the extent of 7 to 10 parts by weight in their preferred solvents compared with a solubility of only 1 part by weight of a typical commercially available color-former in their preferred solvents. The inventors were motivated because: "The mentioned solvents for carbonless paper are occasionally viewed with distrust, causing skin irritations upon use of the carbon paper [sic]manufactured therewith." (column 1, lines 24-27) and "Accordingly, there is a need to be able to substitute these solvents by such solvents which are less hazardous in this connection." (column 1, lines 31-33).
Of critical importance to the fingerprinting system is the solvent. The solvent for the color-former must satisfy many requirements. It must be nontoxic and not have an unpleasant odor. It must be colorless or nearly colorless. It must be able to dissolve the color-former and provide a medium in which rapid reaction between color-former and developer can occur. It must be stable and not react with either the color-former or developer. It should be absorbed into the substrate without causing blotting or "feathering." Solvents that have traditionally been used for fingerprinting include diarylalkanes, such as phenylxylylethane and phenylethylphenylethane; aromatic hydrocarbons such as alkylnaphthalenes; biphenyls and substituted biphenyls.
A major problem with present inkless fingerprinting systems is the slow rate of development of the image (i.e., slow speed). This is because inkless fingerprinting systems have traditionally been based on carbonless paper chemistry, and employed color-formers, solvents, and developers designed for carbonless papers. However, unlike carbonless paper systems where initial development is important but ultimate image density develops only after several hours; a fingerprinting system requires rapid development of a good, dark fingerprint within seconds. This is to allow examination of the fingerprint and re-fingerprinting if necessary. Quite often with previous inkless fingerprinting systems, the color of the fingerprint did not appear to develop so the finger was again placed on the developer. Later examination indicated overlapping fingerprints. This, of course, is unacceptable. Similarly, the use of encapsulation solvents for carbonless paper is not favored as carbonless paper solvents do not necessarily satisfy the requirements for a good fingerprinting solvent. Some irritate the skin. Some are expensive, some do not dissolve enough color-former to provide an intense color, and some are too volatile. A particular shortcoming of encapsulation solvents for carbonless paper is that they are "too slow." That is, when used in fingerprinting the image forms very slowly. The need for an improved inkless fingerprinting system remains.